1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions, to methods of making such devices, and to circuits comprising such devices.
2. Introduction to the Invention
Electrical devices comprising conductive polymer compositions are well known. Such devices comprise an element composed of a conductive polymer. The element is physically and electrically connected to at least one electrode suitable for attachment to a source of electrical power. The factors determining the type of electrode used include the specific application, the configuration of the device, the surface to which the device is to be attached, the resistance of the device, and the nature of the conductive polymer. Among those types of electrodes that have been used are solid and stranded wires, metal foils, perforated and expanded metal sheets, porous electrodes, and conductive inks and paints. When the conductive polymer element is in the form of a sheet or a laminar element, metal foil electrodes that are directly attached to the surface of the conductive polymer, sandwiching the element, are particularly preferred. Examples of such devices are found in U.S. Pat. No. 4,426,633 (Taylor), U.S. Pat. No. 4,689,475 (Matthiesen), U.S. Pat. No. 4,800,253 (Kleiner et al), U.S. Pat. No. 4,857,880 (Au et al), U.S. Pat. No. 4,907,340 (Fang et al), U.S. Pat. No. 4,924,074 (Fang et al), U.S. Pat. No. 5,831,510 (Zhang et al), U.S. Pat. No. 5,852,397 (Chan et al), U.S. Pat. No. 5,864,281 (Zhang et al), and U.S. Pat. No. 5,874,885 (Chandler et al), the disclosures of which are incorporated herein by reference.
Metal foils having microrough surfaces can give excellent results when used as electrodes in contact with conductive polymers. U.S. Pat. No. 4,689,475 discloses the use of metal foils that have surface irregularities, e.g. nodules, which protrude from the surface by 0.1 to 100 xcexcm and have at least one dimension parallel to the surface which is at most 100 xcexcm. U.S. Pat. No. 4,800,253 discloses the use of metal foils with a microrough surface which comprises macronodules which themselves comprise micronodules. U.S. Pat. No. 5,874,885 discloses the use of a metal electrode made of more than one type of metal with particular surface characteristics. Other documents which disclose the use of metal foils having rough surfaces, but which do not disclose the characteristics of the foils, are Japanese Patent Kokai No. 62-113402 (Murata, 1987), Japanese Patent Kokoku H4-18681 (Idemitsu Kosan, 1992), and German Patent Application No. 3707494A (Nippon Mektron Ltd., 1988). U.S. Pat. No. 5,880,668 discloses the use of a modified polyolefin with a carboxylic acid derivative grafted onto the polymer in combination with certain foils. The disclosure of each of these documents is incorporated herein by reference.
Desired properties of electrode materials for conductive polymer devices include: a low contact resistance to the polymer, a strong bond which will survive extended and repetitive electrical and/or mechanical stresses and adverse environmental conditions such as extreme temperatures, temperature cycling and heat and humidity; compatibility with conventional fabrication techniques; and low cost.
We have found that improved electroding for conductive polymers can be accomplished by using foil having a combination of surface features making up the surface roughness which is in a range which is lower than that previously used, in combination with an adhesion promoting layer. Until now, the primary mechanism proposed for forming a good bond between conductive polymers and metal foils has been mechanical interlocking achieved by using a rough surface on the metal foil, wherein the surface of the foil is imbedded into the conductive polymer by heating the polymer above its melting point during the electroding process. The resulting devices can have low contact resistance and good electrical performance. However, we have found that improved performance can be achieved by fabricating devices with metal foils that have a surface having a surface roughness parameter described by the product of two characteristic measurements of surface properties, and using an adhesion promoting layer between that surface of metal foil and the conductive polymer. Devices made by the present invention demonstrate low electrical resistance indicating low contact resistance at the electrode-polymer interface, resistance stability following thermal cycling, and improved resistance stability during and following prolonged and repeated electrical stress.
Ra is a measure of one aspect of surface roughness known as xe2x80x9ccenter line average roughness,xe2x80x9d which relates to an average value of the height of protrusions from a surface and is further described below. The measurement of the average value of height of protrusions, however, does not give any information about the density, distribution, or nature of the protrusions (e.g. spiked, rounded, etc.). A measurement of the reflection density RD of the surface (described below) gives a value which relates to the amount of light reflected from a surface using fixed incident light parameters, and thus gives a measure of the amount of structure on the surface on a size scale comparable to the wavelength of the light (i.e., visible light, around 600 nm). A shiny smooth surface will give a low reflection density, as most of the light will be reflected. The combination of Ra and RD can be used to describe the surface of a foil, and it is especially useful to multiply Ra by RD to describe the surface of the foil.
Foils that have lower surface roughness characteristics than those previously used can be less expensive than those with higher surface roughness. In addition, lamination of viscous or highly filled conductive polymer composites using melt processing can be facilitated by the use of lower structure foils since it is easier to imbed features of smaller average height into the viscous composites. For example, a faster line speed can be allowed since less time is required for the polymer to flow around and fill in a structured foil surface. In a foil which has a surface which is relatively rough, it is possible that the conductive polymer composition will not fill in completely around the features of the foil surface, resulting in trapped air pockets which disrupt the electrical continuity and provide points of failure at the interface, especially under electrical stress or environmental aging. As disclosed in copending commonly assigned application Ser. No. 09/626,825, filed contemporaneously with this application, the disclosure of which is incorporated herein by reference, metal foils with a surface parameter described by the product Ra times RD in a certain range make excellent electrodes for conductive polymer elements. We have found that foils with an even broader range of Ra times RD can be used in combination with an adhesion promoting layer. The resulting bond is very strong mechanically, and there is no undesired increase in resistance which can occur when an adhesion promoting layer is used with some foils, forming contact resistance at the interface. Thus, the combination of foil and adhesion promoting layer of this invention provides devices which have low resistance (indicating low contact resistance), stable resistance following electrical stress, and stable resistance following aging. This allows the use of inexpensive low roughness foils which have been previously unusable because of their inability to form a robust bond to conductive polymers, especially highly filled or nonpolar polymers. The cost of production of devices with lower roughness foils can also be reduced because lamination speeds can be increased. Finally, the invention enables the effective lamination of conductive polymer compositions which do not flow effectively, such as compositions which have been crosslinked, or are extremely highly filled.
The use of coupling agents as adhesion promoters is well known. Coupling agents or adhesion promoters are used to make adhesive bonds between polymers and metal or glass. The bonds thus formed are able to withstand conditions such as high temperature, high humidity, and physical stress. For example, such adhesion promoters are widely used in the circuit board industry for adhering thin metal foil to insulating composite substrates. However, in such an application, in which the substrate is insulating, the quality of the electrical contact between the metal and the substrate is not important, as the substrate is functioning as a dielectric. Conversely, when applying electrodes to a conductive polymer for the purpose of making an electrical device, it is important that good electrical contact exist between the metal and the conductive paths within the polymer and that uniform electrical contact exist over the area of the electrodes. Furthermore, the metal-polymer junction must not have a substantially higher resistance than the bulk of the device, otherwise it can be a point of failure. Many approaches have been taken to fabricate conductive polymer devices that do not have a region of high resistance at the electrode/polymer interface. For example, treating the surface by plasma etching to remove a thin polymer rich layer or the use of conductive tie layers or conductive adhesives between the electrode and the conductive polymer is known. The use of amine- and carboxylic-acid containing materials at the electrode-polymer interface has been disclosed in U.S. Pat. No. 3,858,144 (Bedard et al) as a means of stabilizing the resistance of a conductive polymer containing electrical devices.
Coupling agents have also been used in conductive polymer formulations to enhance the interaction between conductive particles or other additives and a matrix polymer. See for example U.S. Pat. Nos. 4,774,024 and 4,884,163 (Deep et al.), the disclosure of which is incorporated herein by reference. It is disclosed in these patents that coupling agents used in the bulk of the material can improve the adhesion of the conductive polymer composite to the electrode. In that instance, however, the coupling agent is not expressly applied to the junction between the electrode and polymer. Japanese Patent No. 2698995 (TDK, 1998) discloses that a coupling agent can be applied to a metal electrode attached to a polymeric PTC element to improve the electrode-polymer bond strength, as measured by peel strength. However, a high peel strength, while indicative of some types of mechanical strength, is not indicative of a quality electrical connection between the electrode and the conductive polymer. That is, peel strength could not give an indication of a device""s electrode-polymer contact resistance or its ability to withstand electrical stress. We have now made devices incorporating the use of a coupling agent in combination with certain metal foil electrode properties to adhere metal foil to a conductive polymer device that does not interrupt the electrical conductivity between the conductive pathways in the polymer material and the electrodes. The resulting device has low resistance indicating low contact resistance, and stable resistance following prolonged and repeated stresses.
In a first aspect this invention provides an electrical device comprising
(A) an element which
(1) has first and second surfaces, and
(2) comprises a conductive polymer, and
(B) a first metal foil electrode which
(1) comprises
(a) a first surface having (i) a center line average roughness Ra xcexcm and (ii) a reflection density RD, such that the product Ra times RD is at least 0.06 xcexcm, and
(b) a second surface, and
(2) is positioned so that the first surface of the first electrode is in contact with the first surface of the conductive polymer element with a first adhesion promoting layer positioned therebetween.
In a second aspect, this invention provides a process for making an electrical device, the process comprising
(A) providing an element comprising a conductive polymer composition,
(B) providing a first metal electrode having
(1) a first surface having a center line average roughness Ra and a reflection density RD such that the product Ra times RD is at least 0.06 xcexcm, and
(2) a second surface,
(C) providing at least one adhesion promoting layer between the conductive polymer and the first surface of the first metal electrode, and
(D) securing the metal electrode to the conductive polymer with the adhesion promoting layer therebetween.
In a third aspect, this invention provides an electrical device comprising
(A) an element comprising a conductive polymer composition,
(B) a metal electrode comprising
(1) a base metal foil and
(2) first and second surfaces, said first surface in contact with the element and comprising
(a) protrusions having a maximum height of 1 xcexcm, and
(b) a reflection density RD of at least 0.6, and
(C) an adhesion promoting layer positioned between the first surface of the electrode and the element.
In a fourth aspect, this invention provides an electrical device comprising
(A) an element comprising a conductive polymer composition,
(B) a metal electrode, the metal electrode comprising
(1) a base metal foil,
(2) a first surface which (a) comprises dendritic metal structures, and (b) is in contact with the element, and
(3) a second surface, and
(C) an adhesion promoting layer positioned between the first surface of the metal electrode and the element.
In a fifth aspect, this invention provides an electrical circuit which comprises
(A) a source of electrical power;
(B) a load; and
(C) an electrical device, e.g. a circuit protection device, of the first, third, or fourth aspect of the invention electrically connecting the source and the load.